1. Field of the Invention
This invention relates to a video character display system and more particularly to a dot matrix character generator capable of selectively positioning a plurality of lines of matrix dots within a row containing a larger number of lines.
2. Prior Art
A common method of generating a plurality of characters such as alpha-numerics, punctuation symbols, and special code symbols on a display device such as a cathode ray tube involves formation of the characters as a rectangular matrix of dots; selective dots within the matrix are energized or illuminated to generate the desired character. A row of these characters are generated across the width of a CRT screen by scanning the screen horizontally a number of times, with successive downward vertical displacements between each line. To generate a full row a number of lines equal to the number of vertical elements in the matrix are generated. During each horizontal scan the luminence of the cathode ray beam is modulated, i.e. the beam is turned on and off, in accordance with the dots in the particular line of the matrix pattern being scanned.
With a five by seven matrix, which is commonly employed in low definition applications, each character is specified by 35 points and 35 bits of information, seven lines of five bits each, are used to define the matrix. The character to be displayed at any instant is defined by a short binary code which specifies the character type but not the matrix. In a system in which 128 different characters may be displayed a seven bit code is employed. The code for a particular character to be displayed is applied to a memory which stores the dot matrix for each of the characters. Typically the memory also receives the number of the line in the matrix which is being generated and outputs a five-bit signal representative of the five dots in the matrix for the particular line of the defined character.
With these dot matrix systems a problem is created by the fact that certain of the lower case letters, typically j, p and q have sections which descend below the normal bottom margin of the row. If the two bottom lines of the matrix are used solely for the display of these descenders the remaining letters are cramped. Type styles which do not include the descenders are difficult to read. If the matrix is enlarged to say 5 .times. 9 or 5 .times. 10 in order to accommodate these descenders the size and cost of the character code matrix memory is substantially increased. For example, a 5 .times. 7 matrix for 128 characters requires about 5,000 bits while a 5 .times. 10 matrix would require about 10,000 bits, doubling the cost of the memory.